Boiler pump speed alternating

[davidev]

I have a Worceseter Greenstar 12i feeding an indirect cylinder and two central heating zones, controlled by a 3 channel programmer. The heating zones have a thermostat each and TRVs on all the radiators except the towel rail. Whenever the system demands heat - to the cylinder, a heating zone, or any combination - the pump speed on the boiler alternates between a higher and lower speed every 5 seconds or so. (At least, I assume that's what's happening - the audible hum from the boiler changes note, ditto audible flow speed in the rads).
Does anyone have any insight into what may be causing this, and how it might be fixed? It's driving me up the wall.